- Return errors from MasqueConn.Read/Write instead of silently
succeeding (prevents infinite busy-loops and silent data loss
if these methods are accidentally called).
- Set recorder Network field in protocol dispatch switch rather
than hardcoding "udp", so early errors don't misreport type.
- Clarify that DatagramConn context-ID handling only supports
context ID 0 (sufficient for CONNECT-UDP per RFC 9298).